The present invention relates to sheet-feed tractors of the type used, for example, in the data-processing field for automatically advancing paper on which data is to be printed. In particular, the present invention is directed to novel means for clamping the sheet-feed tractor on a support shaft.
The typical sheet-feed tractor includes a toothed belt mounted on the tractor body and propelled by a drive sprocket. The drive sprocket is driven by a drive shaft that rotates to cause the drive belt to follow a path around the tractor. In operation, the teeth of the moving drive belt are received in perforations in sheet material and advance the sheet material whenever a new line is to be typed.
Sheet-feed tractors are typically used in pairs, one for each edge of the perforated paper. A support shaft spaced from the drive shaft extends through openings in the tractor bodies, and the tractors are clamped to the support shaft at the spacing necessary to allow the teeth on both belts to register simultaneously with the perforations in the sheet material. The support shaft also prevents the tractor from rotating with the drive shaft.
Because sheet material of different widths may be used, it is desirable that the tractors permit clamping and unclamping to be performed with relative ease. Numerous clamping mechanisms have been proposed to provide this ease of adjustment. These are exemplified, for instance, by U.S. Pat. No. 4,129,239 to Hubbard, which employs a clamp consisting of two members, one of which includes a split ring that surrounds the support shaft, the other of which includes a ring with flats on its interior surface that encircles the split ring and compresses it upon being rotated. Compression of the split ring causes it to bear against the support shaft and thereby clamp the tractor in place. To leave room for tolerances in support shafts and related equipment, clamps of the type described in Hubbard are slidably mounted in the tractor.
Although the arrangements exemplified by the Hubbard patent provide for rapid adjustment of sheet width, they also have features that detract from flexibility and ease of manufacture. In particular, the split-ring portion must be resiliently deflected in use to provide proper clamping. When the materials typically employed are considered, this results in the requirement that the difference in diameter between the open and closed positions of the clamp be relatively small so as to avoid excessive strain and fatigue of the split ring. Relatively close tolerances on the split ring are therefore desirable, but the manufacturing process is not as simple as might be desired. Furthermore, the requirement has resulted that the diameter of the support shaft be kept within a relatively small range so that the clamp can be effective. This requires relatively expensive shafts and reduces the versatility of the tractor.
It is accordingly an object of the present invention to permit fast and easy clamping on shafts whose diameters vary significantly. It is a further object to achieve this effect in a clamp that is subjected to very little resilient deflection in use.